Accept generic float type for fitness/mutation rate

This crate should work with any type that implements `PartialOrd`, not just `f32`.

However, it would add a bit of complexity to the project, and I don't know if that's really worth it since I don't see any reason why someone wouldn't just use `f32`.

ig it could be useful to be able to use `f64` for more precision, but I still don't know whether it's worth making a big refactor for.